Method and device for refining a glass melt using negative pressure

ABSTRACT

The apparatus for reduced-pressure refining of a glass melt includes a refining bank formed so that a reduced pressure is generated by a glass flow in it. The refining bank has a component, which is made from a refractory metal or refractory alloy acting as glass-contact material. The refractory metal or alloy contains molybdenum, tungsten, tantalum, and/or hafnium. The device of the present invention includes a protective gas reservoir for a protective gas and an automatically operating valve connecting the reservoir with the refining bank so that an inner side of the component that would otherwise be exposed when a pressure rise or a falling glass melt column occurs is protected from oxidation by the protective gas. A process for using the device during refining of the glass melt is also part of the invention.

CROSS-REFERENCE

This is the U.S. National Stage of PCT/EP 01/12197, filed Oct. 23, 2001,which, in turn, claims the benefit of priority of invention under 35U.S.C. 119 (a) and 35 U.S.C. 365 (b) based on DE 100 54 881.4 filed Nov.6, 2000 in Germany and based on DE 101 28 674.0 filed Jun. 13, 2001 inGermany.

BACKGROUND OF THE INVENTION

The invention relates to a process and device for the reduced-pressurerefining of a glass melt using a reduced-pressure apparatus, in whichthe glass melt is fed to a refining bank via a riser and is dischargedagain from the refining bank via a downpipe, a reduced pressure beinggenerated by means of the glass flow in the refining bank.

The refining of the glass melt, i.e. the removal of gas bubbles from theglass melt, is used to eliminate bubbles. With small crucible melts, therise of the gas bubbles out of the glass melt has already beenaccelerated by means of the application of a reduced pressure above theglass melt.

Devices for reduced-pressure refining of a glass melt using areduced-pressure apparatus of the type mentioned in the introduction areknown, for example, from documents U.S. Pat. No. 1,598,308, EP 0 908 417A2 and JP 2-2211229 A. The first two of these documents disclose the useof ceramic refractory materials as glass-contact material, while thelast of these documents discloses the use of platinum or platinum alloysas glass-contact material.

Both the use of ceramic refractory materials and the use of platinum andits alloys are associated with a range of drawbacks.

For example, ceramic refractory materials, when in contact with a glassmelt, are subject to considerable wear and increased corrosion comparedto platinum and its alloys. This entails firstly short plant operatingtimes and a high outlay on maintenance and repair and secondly a highpotential for the formation of glass defects (formation of cords,inclusions, bubbles in particular in the downpipe). Furthermore, heatingpresents problems for glass melts which cannot be heated directly byelectrical means or can only be heated in this way with difficulty.

With regard to the formation of glass defects, it is preferable to useplatinum or platinum alloys, but the capital costs which this incurs arevery high.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide a device and aprocess for the reduced-pressure refining of a glass melt in which thedrawbacks mentioned in the introduction are overcome. With regard tocontact with the glass melt which is to be refined, the device is to becorrosion-resistant and wear-resistant, is to bellow-maintenance and isto have the minimum possible procurement and operating costs.Furthermore, it is to be possible to heat the glass melt inside thedevice, in particular inside the riser and downpipe.

This object is achieved when the device for the reduced-pressurerefining of a glass melt using a reduced-pressure apparatus in which theglass melt is fed to a refining bank via a riser and is discharged againfrom the refining bank via a downpipe, a reduced pressure beinggenerated by means of the glass flow in the refining bank, is designedin such a manner that the riser and/or the downpipe and/or the refiningbank comprises at least one component made from at least one refractorymetal and/or from a refractory metal alloy acting as glass-contactmaterial.

In the process according to the invention for the reduced-pressurerefining of a glass melt in a reduced-pressure apparatus, in which theglass melt is fed to a refining bank via a riser and is discharged againfrom the refining bank via a downpipe, a reduced pressure is generatedby means of the flow of glass in the refining bank, and the riser and/orthe downpipe and/or the refining bank are provided with at least onecomponent made from at least one refractory metal and/or from arefractory metal alloy as glass-contact material.

Since the riser and/or the downpipe and/or the refining bank has atleast one component made from at least one refractory metal and/orrefractory metal alloy as glass-contact material, it is possible tosatisfy the requirements which were described when the objectives werebeing set.

It has been possible to determine that components made from molybdenum,tungsten, tantalum or hafnium or from an alloy which contains at leastone of these refractory metals satisfy the abovementioned demandsimposed on the device for reduced-pressure refining particularly well.Tantalum and hafnium and alloys thereof in particular form oxidicprotective layers with a low vapor pressure.

To protect the components—refractory metals and their alloys aregenerally oxidized considerably at temperatures of over 500° C. in thepresence of oxygen—that side of the components which is remote from theglass melt is preferably flushed with a protective gas (e.g. nitrogen,noble gas) or forming gas (hydrogen/nitrogen, noble gas), oralternatively the side remote from the glass melt is protected fromoxidation by glazing, for example by a deliberate flow of the glass meltbehind the components. By contrast, that side of the components which isin contact with the glass melt is sufficiently protected by the meltwhile the device is operating. For the flushing, at least the componentis located in a housing which has a feed for the protective gas orforming gas and a corresponding discharge. The hydrogen content of theforming gas may be up to 100%.

To ensure optimum operation of the device, the components are ofvacuum-tight design and are preferably also designed to be mechanicallystable with respect to pressure differences.

In a particularly preferred embodiment of the device, the componentscomprise individual sections, in particular pipe sections, which areconnected to one another by means of a flange connection or a screwconnection. Pipe sections can be produced and processed further easilyand inexpensively from refractory metals or their alloys. Theconnections between the pipe sections can be made gas-tight in aparticularly simple way by means of cutting edges. An additional sealingaction is achieved if the pipe sections which have been connected to oneanother are annealed, preferably at temperatures of over 1000° C.,during which step the contact locations are welded or sintered together.

The cutting edge seal is a static connection which is free of sealingmaterial. It is used here to connect rotationally symmetrical pipesections. To increase the pressure, at in each case one of the pipepairs which are to be connected the sealing surface (planar bearingsurface) is reduced in size in such a manner that concentricallyencircling cutting edges which are V-shaped in cross section are formed.When the connecting locations are clamped together, the pressure on thecutting edges is increased to such an extent that, if necessary, thematerial can flow.

As has already been mentioned, the component is preferably located in ahousing, in particular in a gas-tight housing, with the thermalexpansion of the component with respect to the housing being compensatedfor, for example by a spring-assisted bellows which is preferably partof the housing. The inner components which carry glass melts are subjectto higher temperatures than the surrounding parts and accordingly expandto a greater extent.

The gas pressure inside the housing should be slightly increasedcompared to atmospheric pressure. If a reduced pressure prevails in thehousing, there is the risk of an explosive mixture with the forming gasbeing formed as a result of air being sucked in through leaks. Air beingsucked in would also lead to considerable oxidation of the component onthe rear side. However, this means that the component must additionallyhave a high mechanical stability in order to be able to withstand (bear)the pressure difference (slightly increased pressure on the outer side,reduced pressure on the inner side). Also, all the connections must bedesigned to be gas-tight. Since a slight excess pressure is used in thegas space, it is imperative that no protective gases pass into the glassmelt. This would lead to the formation of glass defects (formation ofgas bubbles in the glass, reduction of glass constituents in the case ofhydrogen-containing protective gas).

Furthermore, it is advantageous if the component can be heated.Particularly in the case of devices for reduced-pressure refining ofglass melts which have a high surface to volume ratio of the glass meltand/or a low flow rate and therefore a high dissipation of heat to theenvironment, it is necessary for the glass melt to be additionallyheated. In this context, one possible option is heating by means ofradiation heaters (e.g. grids of refractory metal or alloys thereof,e.g. of molybdenum, tungsten, tantalum or hafnium), the radiationheaters—since they reach surface temperatures of up to 2200° C. duringoperation—if necessary being protected against oxidation by beingflushed with a protective gas or forming gas or by glazing in a similarmanner to the components.

Furthermore, the heating of the components may also be effected byinduction or by direct flow of high-frequency alternating current in thecomponent. Heating by means of direct flow of current in the glass meltusing a central stick electrode and the component as a counterelectrodeis also advantageous in particular if the glass melt has a sufficientelectrical conductivity.

In order, in particular in the event of a pressure rise or a drop in theglass melt column, for the inner side of the component—which is then nolonger covered by the glass melt—to be protected from oxidation, thedevice can be at least partially flooded with a protective gas. In thiscase, there is in particular a connection to a protective-gas reservoirfor flooding the device with a protective gas, in particular anautomatic connection, for example a valve which opens automatically whenrequired.

BRIEF DESCRIPTION OF THE DRAWING

The invention is to be explained in more detail with reference to thefollowing exemplary embodiments and drawings, in which:

FIG. 1 shows a detailed view of a device according to the invention forreduced-pressure refining with radiation heaters,

FIG. 2 shows a detailed view of a device according to the invention forreduced-pressure refining with inductive heating,

FIG. 3 shows a detailed view of a device according to the invention forreduced-pressure refining with direct electrical heating of the glassmelt, and

FIG. 4 shows a detailed view of a device according to the invention forreduced-pressure refining with spring-assisted bellows for compensatingfor the thermal expansions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows part of a device according to the invention for thereduced-pressure refining of a glass melt using a reduced-pressureapparatus in which the glass melt (2) is fed to a refining bank via ariser and is discharged again from the refining bank via a downpipe, areduced pressure being generated by means of the glass flow in therefining bank and the riser and/or the downpipe and/or the refining bankhaving at least one component (1) made from at least one refractorymetal and/or from a refractory metal alloy as glass-contact material.

The component (1), which is of tubular design and is preferably madefrom molybdenum or tungsten or a corresponding alloy, is surrounded by ahousing (10) (e.g. made from steel, aluminum or plastic). The feed (3)and discharge (4) for the protective gas or forming gas are also locatedin the housing.

On the side remote from the glass melt, the component (1) is protectedfrom oxidation by flushing with a protective gas or forming gas, and theradiation heater (5) is also protected.

Refractory material (6) is located between the radiation heaters (5) andthe housing. The role of the refractory material is firstly to minimizethe heat losses and secondly to reduce the temperature to such an extentthat the housing material is not damaged. Under certain circumstances,for this purpose individual regions are to be provided with cooling, forexample water cooling. Lead-throughs (not shown), which enable theprotective gas to flow to and from the component and the radiationheaters, are provided in the refractory material.

The radiation heaters are arranged around the component in such a waythat they can directly irradiate or heat the component. The electricalfeeds to the heaters are protected against the high temperatures (e.g.by water cooling and by screening the radiation).

FIG. 2 likewise shows a detailed view of a further device according tothe invention. In this case too, a refractory material (6) is arrangedaround a tubular component (1), with a gap-through which the protectivegas or forming gas can flow remaining between the component andinsulation. The induction coil (8) (which may be water-cooled and/oradditionally thermally insulated) for inductive heating of the componentis arranged in the cooler region between the housing and the insulation.

FIG. 3 likewise shows a detailed view of part of a device according tothe invention, the glass melt (2) being heated by means of directelectrical heating using a central stick electrode (12) and thecomponent (1) as a counterelectrode. In this embodiment, the heating iseffected by means of the flow of current in the glass melt. Unlike thestandard structure of electrode heating circuits, however, in this casethe heating circuit length is very short on account of the arrangementcomprising the central stick electrode and the component as acounterelectrode, so that even glass melts with a poor electricalconductivity can still be heated using acceptable voltages.

The thermal expansion of the component with respect to the housing iscompensated for, as shown in FIG. 4, by means of a spring-assisted (15)bellows (14) which is part of the housing.

On account of the low electrical resistance of metals, direct electricalheating of a component provided in accordance with the invention usingalternating current at normal mains frequency (50 or 60 Hz) is generallynot practical, on account of the high currents required. However, athigh alternating current frequencies what is known as the skin effectoccurs, causing the current to flow only in a thin layer (skin) at theouter edge of the tube. This is associated with an increase in theelectrical resistance, and consequently the current required iscorrespondingly lower. This means that direct electrical heating of thetube is practical again.

The structure of direct electrical heating of this type corresponds tothat shown in FIG. 2, but without an induction coil, or to the structureshown in FIG. 3, but without a stick electrode. The electricalresistance to direct current for a tubular component with an externaldiameter of 300 mm rises by approximately a factor of 21 at analternating current frequency of 10 kHz and by approximately a factor of67 at an alternating current frequency of 100 kHz.

The components have to be reliably protected against oxidation both whenthe installation is being started up and in emergency situations. Whenthe installation is first being started up, the components can beincorporated coated on the inner side with a commercially availableoxidation-resistant coating (e.g. SIBOR produced by PLANSEE). Duringoperation, this coating dissolves in the glass melt. Another possibilityconsists in the empty volume in the installation being filled with aninert gas (noble gases, nitrogen) or a reducing atmosphere (e.g. byadmixing hydrogen).

In the event of a pressure rise (whether unintended or deliberate) inthe refining bank, causing the glass column to drop to such an extentthat the components are no longer protected from oxidation by the glassmelt, other measures have to be used to protect the component. This canbe achieved by flooding the installation with inert gas or a reducingatmosphere. If the pressure rise is brought about deliberately, this canbe controlled directly via the protective gas inlet.

If the pressure rise is unforeseen (e.g. as a result of a pump failure),the installation must be flooded automatically, for example by asolenoid valve (9), which opens automatically and connects the refiningbank to a protective gas reservoir (7).

1. A device for reduced-pressure refining of a glass melt using areduced-pressure apparatus in which the glass melt is fed to a refiningbank via a riser and is discharged again from the refining bank via adownpipe so that a reduced pressure is generated by a glass flow in therefining bank, wherein said refining bank, said riser, and/or saiddownpipe comprises at least one component, said at least one componentis made from a refractory metal or a refractory metal alloy acting asglass-contact material, and said refractory metal or said refractorymetal alloy contains at least one of said molybdenum, said tungsten,said tantalum, and said hafnium; wherein said device comprises aprotective gas reservoir and an automatic connection for connecting saidprotective gas reservoir to supply a protective gas to protect an innerside of said at least one component from oxidation in the event of apressure rise or in the event of a falling glass melt column.
 2. Thedevice as defined in claim 1, wherein said automatic connectioncomprises an automatically operable valve that automatically opens inresponse to said pressure rise or in response to said falling glass meltcolumn so that said protective gas is automatically supplied to protectsaid inner side of said at least one component.
 3. The device as definedin claim 1, wherein said at least one component consists of saidrefractory metal.
 4. The device as defined in claim 1, wherein said atleast one component comprises said tantalum or said hafnium.
 5. Thedevice as claimed in claim 1, wherein a side of said at least onecomponent remote from another side of the at least one componentcontacted by said glass melt is protected by purging with saidprotective gas or a forming gas.
 6. The device as claimed in claim 1,further comprising glazing a side of said at least one component remotefrom the glass melt in order to protect said at least one component fromoxidation.
 7. The device as claimed in claim 1, wherein said at leastone component is of a vacuum-tight design.
 8. The device as claimed inclaim 1, wherein said at least one component is mechanically stable withrespect to pressure differences.
 9. The device as claimed in claim 1,wherein said at least one component comprises individual pipe sectionsand said individual pipe sections are connected to one another by aflange connection or a screw connection.
 10. The device as claimed inclaim 9, wherein said flange connection or said screw connection is madegas-tight by means of cutting edges.
 11. The device as claimed in claim9, wherein said individual pipe sections connected to one another areannealed at high temperatures, so that contact locations between thepipe sections are welded or sintered together.
 12. The device as claimedin claim 1, wherein said at least one component is located in a housing.13. The device as claimed in claim 12, wherein said housing isgas-tight.
 14. The device as claimed in claim 12, further comprisingmeans for compensating for thermal expansion of said at least onecomponent with respect to said housing.
 15. The device as claimed inclaim 14, wherein said means for compensating for thermal expansioncomprises a spring-assisted bellows and said housing comprises saidspring-assisted bellows.
 16. The device as claimed in claim 1, furthercomprising means for heating said at least one component.
 17. The deviceas claimed in claim 16, wherein said means for heating said at least onecomponent comprises at least one radiation heater.
 18. The device asclaimed in claim 17, wherein said at least one radiation heater isprotected from oxidation by flushing said at least one radiation heaterwith said protective gas or a forming gas.
 19. The device as claimed inclaim 17, wherein said at least one radiation heater is protected fromoxidation by glazing.
 20. The device as claimed in claim 16, whereinsaid means for heating comprises means for inductive heating said atleast one component.
 21. The device as claimed in claim 16, wherein saidmeans for heating said at least one component comprises means forpassing a high-frequency alternating current through said at least onecomponent.
 22. The device as claimed in claim 16, wherein said means forheating said at least one component comprises means for providing adirect flow of current in said glass melt between a central stickelectrode and said at least one component, wherein said at least onecomponent acts as counter electrode.
 23. A process for reduced-pressurerefining of a glass melt in a reduced-pressure apparatus comprising arefining bank for the glass melt, a riser for supplying the glass meltto the refining bank, and a downpipe for discharge of the glass meltfrom the refining bank, wherein said refining bank, said riser, and/orsaid downpipe comprises at least one component made from at least onerefractory metal or a refractory metal alloy acting as glass-contactmaterial, said at least one refractory metal is selected from the groupconsisting of molybdenum, tungsten, tantalum, and hafnium, and saidrefractory metal alloy contains at least one of said molybdenum, saidtungsten, said tantalum, and said hafnium, so that said at least onecomponent contacts the glass melt; said process comprising the steps of:a) feeding a glass melt to be refined to the refining bank via theriser; b) generating a reduced pressure in the refining bank by means ofa glass flow in the refining bank; and c) supplying a protective gasfrom a protective reservoir via an automatic connection to protect aninner side of the at least one component from oxidation when a pressurerise or a fall of a glass melt column occurs.
 24. The process as definedin claim 23, wherein said refining bank includes said at least onecomponent and said automatic connection comprises an automaticallyopening valve that opens to supply said refining bank with saidprotective gas in the event of said pressure rise or said fall of theglass melt column.
 25. The process as defined in claim 23, wherein saidprotective gas comprises an inert gas or a reducing gas.
 26. The processas defined in claim 23, wherein said at least one component consists ofsaid refractory metal.
 27. The process as defined in claim 23, furthercomprising heating said at least one component during said refining. 28.The process as defined in claim 27, further comprising compensating forthermal expansion of said at least one component during said heating.